Low carbon ferritic stainless steel



Feb. 27, 1962 R. D. WYLIE 3,023,093

LOW CARBON FERRITIC STAINLESS STEEL FiledNov. 3, 1958 2 sheets-sheet i STRESS PSI TENSILIE STRIEN YIELD STRENGTH ,02 OFFSET GE] A INDIVIDUAL HEATS TEMPERATURE F SHORT TIME- men TEMPERATURE- TENSILE TEsTs 0N INVENTION ALLOYS F I G. 1

IN V EN TOR.

ATTORN EY Feb. 27, 1962 R. D. WYLIE LOW CARBON FERRITIC STAINLESS STEEL 2 Sheets-Sheet 2 Filed Nov. 3, 1958 mmDhmDm Oh mmDOI 009 O9 I56 NI $83818 IN VEN TOR. 05m .0 h g/z'e gflm ATTORNEY United St tes 3 023 008 LOW CARBON FEliRl'i lC STAINLESS STEEL Robert D. Wylie, Barberton, Ohio, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Nov. 3, 1958, Ser. No. 771,392 Claims. (Cl. 75-125) be subjected to contact with corrosive liquid or vapor either simultaneously or in alternation.

In many heat exchanger installations, certain factors, such as high temperatures and stress, for example, dictate the use of austenitic stainless steels. Such steels are used, for example, in nuclear power generating equipment. However, it has been noted that there is a tendency for austenitic stainless steels of the chromium-nickel type to be subject to stress corrosion cracking, particularly when such steels are in contact with aqueous media containing very small concentration of chloride ions, and particularly in zones where liquid or vapor are either coexisting or are alternately present. Stresses in these austenitic stainless steels, due to tube expanding, welding, and operating conditions, have been known to produce failures.

The amount of austenitic stainless steel alloys required in steam generating units has been increasing as design specifications have become more demanding. Due to the cost of these materials, investigations have been aimed toward the development of alloys having equivalent or better physical and mechanical properties but which are more econimical in first cost than the austenitic stainless steels. This demand for more economic alloys has been accentuated by the stringent requirements of nuclear reactor design as well as of conventional design fossil fuel-fired steam generating units.

Heat exchangers for certain special propulsion systems require an unusual degree of integrity under operating conditions which are much more severe than those usually encountered. These heat exchangers must have welded connections between the tube and tube sheets, and these welded connections result in configurations which are susceptible to stress corrosion. Recent developments have indicated that failures inthe general area of the welded connections, may be associated with stress corrosion, which may be aggravated by thermal shock. It further appears that austenitic stainless steels are particularly susceptible to this type of failure.

On the other hand, chromium stainless steels of the ferritic or martensitic type, while substantially less expensive than the austenitic stainless steels, have not found favor for high temperature, high pressure installations due to the mechanical properties of these ferritic steels and the effects of fabrication on these properties. However, it is known that these ferritic chromium stainles steels have a greater resistance to stress corrosion cracking, particularly of the type occurring when the steels are sub jected with liquids and vapors including small concentrations of chloride ions.

In accordance with the present invention a ferritic stainless steel is provided which is easy to fabricate into articles such as tubing by conventionalapparatus and procedures, as compared to the difiiculties and expense encountered in fabricating austenitic stainless steels,-has high tensile, yield and stress-rupture strengths properties at moderately elevated temperatures, excellent elongation .and ductility properties, and stress corrosion resistance properties exceeding those of austenitic stainless steels. This ferritic stainless alloy has a nominal composition of Cr and 1 Ni with a carbon content not exceeding 3,023,098 Patented Feb. 27, 1982 C 0.03 max. (ladle) 0.035 max. (check). Cr 14.00 to 17.00.

Ni 1.00 to 1.50.

Mn 0.30 to 0.90.

Si 0.75 max.

S c 0.03 max.

P 0.03 max.

Cu -c 0.00 to 1.10.

Mo 0.00 to 0.60.

balance iron with the usual impurities. Where Cu is added, the percentage by weight is preferably in the range from 0.90 to 1.10 and, where M0 is added, the percentage by weight is preferably in the range from 0.40 to 0.60.

Within this range are ferritic stainless steel alloys hav ing percentage by weight compositions as follows:

Alloy No 1 2 a 1 5 0 o. 51 o. 74 o. 0. 67 0. 7s

in each alloy,- the balance is iron with the usual impurities.

A preferred ferritic stainless steel alloy embodying the invention has the following percentage composition by weight:

C 0.03 max. .(ladle) 0.035 max.

- (check).

Cr 14.00 to 16.00.

Ni 1.00 to 1.50.

Mn 0.75 max.

Si 0.50 max.

S 0.030 max.

P 0.030 max.

Tensile strength in p.s.i. an; Yield strength (0.2% offset) in p.s.i. Percent elongation in 2 Hardness Spreads, Tensile strength in p.s.i. 8 0,370-401,500- Yield strength (0.2% otfset) in p.s.i. S7,93083,000 Percent elongation in 2" 25-310 Percent reduction of area -65.681.1

The characteristics or the invention alloy steels when subjected to tension for short intervals at high temperatures are illustrated in FIG. 1 of the drawing. The graph of FIG. 1 was prepared from the following table representing these different characteristics of the alloy steels:

Yield Tensile Test strength strength Percent Percent Nurn- Numtemp., 02% oiiset X1,000 elong. in red. of her of her-f 1,000 p.s.i. 2" area heats tests p.s.i

FIG. 2 graphically illustrates the stress rupture properties of a wrought bar of an alloy incorporating the invention, when subjected to normalizing at 1700 F., followed by air cooling, drawing at 1300 F., and furnace cooling. The stress rupture values of FIG. 2 are all taken at 900 F. and from the following table:

Percent Percent Applied stress, p.s.i. Rupture time, hours elongation reduction in 2" of area 35,000 257.2 40. 5 80. 1 32,000 1,666 41. 5 74. 5 30,000 5,400 to date 28,000 3,450 removed The following Charpy V-notch impact values are from various alloys embodying the invention which have been given certain heat treatments after forging and/ or rolling operations:

(a) Forged and rolled:

Test temp, F.: Values observed, ft-lbs.

(b) Normalized at 1700 F., drawn at temperature from 1150 F. to 1400 F., tested at room temperature.

23 specimens-values range from 83 to 110 ft.-lbs.

with an average of 92.5 ft. lbs. (0) Normalized at 1700" F., drawn at 1300 F., stress relieved at 1350" F.:

Test temp, F.: Ft.-lbs. impact values Room temp. 50,55 +10 31,27, 55

After 1000 hours at 885 F. as an embrittlement test the following values were observed:

Room temp. 38, 40, 44 +10 36, 37, 38

The alloy steels of the invention have also been tested in the form of weld deposits having compositions according to the invention. The following is a tabulation of the properties of the weld deposits:

As welded After various stress relief Tensile strength, 10.5.1 101, 000-115, 000 S7, 000-120,000 Yield strength (0.2% oliset), psi. 44, 500-100, 00 65, 000-114, 000 Percent elongation in 2".-." 1. 1-3. 0 16-21. 5 Percent red. of area 1. 1-1. 4 35. 9-65. 2 Weld metal impact (Charpy V- notch test) ft. lbs 0-6 Insofar as corrosion resistance of the invention alloys is concerned, the corrosion rate in high purity water is less than that of carbon steel but somewhat greater than thatof Type 347 austenitic stainless'steels. In oxygenated water tests, the corrosion rate of the invention steels is very much better than that of carbon steels. However, when subjected to chloride containing water, the invention alloys have a stress corrosion resistance very much better than that of Type 347 and Type 304 anstenitic stainless steels. Typical examples of these tests are as follows:

(l) MgCl media (a) Conditions:

60 weight percent MgCl .6I-I O Boils about 220 F.

35,000 p.s.i.

Tension type-bar specimen 380 hours Results:

Type 347--severely cracked Invention alloys-no cracking, no pitting Conditions: 7

42% MgCl by weight 309 F. boiling point Horseshoe bend specimens 75% of yield after initial bending Number Treatment of Results samples Hot rolled 7 6 cracked; l uncrecked at 500 hrs. Annealed (after beuding)- 2 2 uncracked at 1,025 hrs. Normalized and tempered 14 2 cracked; 4 uncracked at (after bending). 500 hrs; 8 uneracked at 1,000 hrs. Normalized, tempered and 6 6 uncracked at 500 hrs.

stress relieved (norm. and temp. before bending, S.B. after bending).

(2) Simulated boiler water tests Results:

Type 347 13 U bends ..13 cracked 12 expanded tubes 12 cracked Invention alloys 32 U bends N0ne cracked 10 expanded tubes None cracked RemarksType 347 exhibits typical stress corrosion cracks.

Invention alloy exhibits some pitting tendencies. However, certain passivation treatments re sulted in excellent pitting resistance.

Metallograph'ic examination of the invention alloy in the form of tubing and extruded bar-stock, and in the process annealed condition, reveals a fine grained microstructure of tempered martensite and ferrite. The microstructure contains approximately 50 percent martensite, which enhances the mechanical properties of the alloy, such as tensile strength and hardness properties, and increases its impact strength, particularly as compared to alloys having a microstructure which is preponderantly ferrite.

While specific embodiments of the invention have been shown and described in detail to illustrate the applica tion of the invention principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. An alloy steel resistant to stress corrosion cracking when subjected to corrosive atmospheres and having good impact strength at low temperatures, excellent mechanical properties at moderately elevated temperatures, and characterized by a fine-grained micrcstructure of tempered martensite and ferrite, the martensite constituting approximating 50 percent of the microstructure; said alloy steel having the following percentage composition by weight:

Cr 14.00 to 16.75 Ni 1.02 to 1.50 C 0.018 to 0.035 Mn 0.51 to 0.90 Si 0.24 to 0.50 S 0.030 max. P 0.030 max. Cu Up to 0.90 Mo Up to 0.51

balance iron with the usual impurities.

2. An alloy steel resistant to stress corrosion cracking when subject to corrosive atmospheres and having good impact strength at low temperatures, excellent mechanical properties at moderately elevated temperatures, and characterized by a fine-grained microstructure of tempered martensite and ferrite, the martensite constituting approximately 50 percent of the microstructure; said alloy steel having the following percentage composition by weight:

Cr. 16.75 Si 0.24 Ni 1.09 S 0.023 C 0.032 P c 0.016 Mn 0.71 Mo 0.51

balance iron with the usual impurities.

3. An alloy steel resistant to stress corrosion cracking when subjected to corrosive atmospheres and having good impact strength at low temperatures, excellent mechanical properties at moderately elevated temperatures, and characterized by a fine-grained microstructure of tempered martensite and ferrite, the martensite constituting approximately 50 percent of the microstructure; said alloy steel having the following percentage composition by weight:

Cr 16.50 Si 0.25 Ni 1.02 S 0.021 C 0.022 P 0.022 Mn 0.51

balance iron with the usual impurities.

4. An alloy steel resistant to stress corrosion cracking when subjected to corrosive atmospheres and having good impact strength at low temperatures, excellent mechanical properties at moderately elevated temperatures, and characterized by a fine-grained microstructure of tempered martensite and ferrite, the martensite constituting approximately 50' percent of the microstructure; said alloy steel having the following percentage composition by weight:

Cr "15.84 Si 0.31 Ni 1.42 S 0.017

Mn 0.74 Cu 0.90

balance iron with the usual impurities.

5. An alloy steel resistant to stress corrosion cracking when subjected to corrosive atmospheres and having good impact strength at low temperatures, excellent mechanical properties at moderately elevated temperatures, and characterized by a fine-grained microstructure of tempered martensite and ferrite, the martensite constituting approximately 50 percent of the microstructure; said alloy steel having the following percentage composition by weight:

Cr 15.72 0.44 Ni 1.16 s 0.008 C 0.024

, P 0.010 Mu 090 Mo 0.49

balance iron with the usual impurities.

'6. An alloy steel resistant to stress corrosion cracking when subjected to corrosive atmospheres and having good impact strength at low temperatures, excellent mechanical properties at moderately elevated temperatures, and characterized by a fine-grained microstructure of tempered martensite and ferrite, the martensite constituting approximately 50 percent of the microstructure; said alloy steel having the following percentage composition by weight:

Cr 15.37 S 0.016

Ni 1.09 P 0.010 C 0.026 Cu 0.10

Mn 0.67 Mo 0.09 Si 0.41

balance iron with the usual impurities.

7. An alloy steel resistant to stress corrosion cracking when subjected to corrosive atmospheres and having good impact strength at low temperatures, excellent mechanical properties at moderately elevated temperatures, and characterized by a fine-grained microstructure of tempered martensite and ferrite, the martensite constituting approximately 50 percent of the microstructure; said alloy having the following percentage composition by weight:

balance iron with the usual impurities.

9. An alloy steel resistant to stress corrosion cracking when subjected to corrosive atmospheres and having good impact strength at low temperatures, excellent mechanical properties at moderately elevated temperatures, and characterized by a fine-grained microstructure of tempered martensite and ferrite, the martensite constituting approximately 50 percent of the microstructure; said alloy steel having the following percentage composition by weight:

Cr 14.0 to 16.0

1.0 to 1.5 0.035 max.

0.75 max. 0.50 max. S 0.030 max. P 0.030 max.

balance iron with the usual impurities.

10. An alloy steel resistant to stress corrosion cracking when subjected to corrosive atmospheres and having good impact strength at low temperatures, good weldability, excellent mechanical properties at moderately elevated 14.0 to 16.0 V 1.0 to 1.5 0.035 max. 0.75 max. 0.50 max. 0.030 max. 0.030 max.

'7 temperatures, and characterized by a microstructure of martensite and ferrite; said alloy steel having the following percentage composition by weight: I

Cr 14.00 to 17.00 Ni 1.00 to 1.50 C 0.035 max. Mn 0.30 to 0.90 Si 0.75 max. S 0.030 max. P 0.030 max. Cu up to 1.10 Mo up to 0.60

and balance iron with the usual impurities.

3 References Cited in the file of this patent UNITED STATES PATENTS 1,441,479 Bennett Jan. 9, 1923 1,630,448 Oertel May 31, 1927 1,775,615 Succop Sept. 9, 1930 2,310,341 Arness Feb. 9, 1943 2,362,277 Jones Nov. 7, 1944 2,851,384 Waxweiler Sept. 9, 1958 OTHER REFERENCES Metal Handbook, 1954 Supplement, pages 34-69; published by The American Society of Metals, Cleveland, Ohio (first published as the July 15, 1954, issue of Metals Progress). 

10. AN ALLOY STEEL RESISTANT TO STRESS CORROSION CRACKING WHEN SUBJECTED TO CORROSIVE ATMOSPHERES AND HAVING GOOD IMPACT STRENGTH AT LOW TEMPERATURES, GOOD WELDABILITY, EXCELLENT MECHANICAL PROPERTIES AT MODERATELY ELEVATED TEMPERATURES, AND CHARACTERIZED BY A MICROSTRUCTURE OF MARTENSITE AND FERRITE; SAID ALLOY STEEL HAVING THE FOLLOWING PERCENTAGE COMPOSITION BY WEIGHT: 